This invention relates to an instrument for monitoring and detecting the health or well being of critical valves, and in particular the health of electromechanical valves such as those used in nuclear power plants.
Electromechanically operated valves are extensively used in industrial processes and in other applications in which their failure can cause serious damage or even be catastrophic regarding property and life. For example, in the control systems of power generation and in particular the cooling of equipment such as nuclear power generators, chemical plants utilizing inflammable and/or hazardous materials, and in the control systems of aircraft, the failure of one or more electrically actuated valves at a critical period could lead to disaster. For example, in a nuclear power plant, control valves are frequently utilized for safety systems such as control of cooling water to prevent nuclear meltdown. In flight control systems valve actuation provides automatic control, for example, of a high speed fighter aircraft. Failure of a control valve in such systems to respond to a control signal obviously could result in serious consequences.
While it is common in critical control systems to utilize redundancy, a duplicate or redundant stand-by control system or subsystem for use in the unlikely case of failure of the primary system is not only expensive and complex, it requires means to detect the failure of the primary system followed by actuation of the redundant or backup system after first switching the redundant system into the control loop of the control system while also disconnecting the malfunctioning primary system. The additional equipment and complexity in such switching and substitution not only introduces a time delay into obtaining corrective action of the control system, but also introduces additional equipment including the switching equipment which in itself can malfunction.
It is accordingly highly desirable to be able to monitor the health of electromechanical valves in critical systems such as nuclear control systems and to detect potential future failures of critical valves in advance of failure, to enable replacement of the unhealthy valve prior to failure in order to assure continuing prompt and proper response of the control system. This does not mean that the primary control system cannot include redundancy backup, but rather contributes to increased safety and improved performance of the overall system by maintaining the primary control system in operation by enabling replacement in advance of failure of those critical valves which exhibit an unhealthy control response and a potential for failure.
In analyzing the health and performance of an electromechanical valve, it is possible to analyze the valve operation by comparing or correlating the actual valve response signal with a normal valve response signal. In order to detect inchoate problems to enable predictive and preventative maintenance, it is desirable to be able to detect and analyze variations in the time behavior of the valve, that is deviations from normal valve response operation. Such variations can be caused by sticking of the valve, or by excess friction and can be used as an indication of the health of the valve and the desirability to replace the abnormally operating valve prior to an actual failure even though the valve is still opening and closing in response to the input control signal.
However, electromechanical valves in applications such as nuclear power plants and in the environments described above operate in a power and control circuit environment in which the background noise is not Gaussian noise, but rather is pulsed noise such that cross-correlation methods of signal analysis is not very effective since its operation is based on an additive white noise Gaussian environment. Since the noise from electrical control and machinery systems is different, and often includes spikes, cross correlation is not necessarily an effective method for good signal discrimination. Noise and electrical disturbance or interference which may exceed any predetermined threshold value can cause false failure indications in cross-correlation methods. For the same reason, a matched filter does not provide good signal processing for such control systems and traditional optimum estimates are not optimal in the presence of noise which is other than Gaussian.
However, instruments to monitor the health of critical valves must be relatively inexpensive and easy to install, not only on new control systems but in retrofitting existing control systems. In addition, it is highly desirable that a single apparatus be able to periodically monitor a plurality of valves in order to minimize the cost and complexity of the monitoring.